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Stacked herbicide resistance progresses across Midwest

Wisconsin State Farmer
Published 12:26 a.m. CT March 28, 2017

America's farmers and ranchers can represent agriculture in the U.S.(Photo: File photo)

INDIANAPOLIS - For nearly two decades, herbicide-resistant weeds have swept across the Midwest. Today, more fields than ever are facing weeds with stacked resistance to multiple modes of action. With new postemergence technologies entering the market, farmers need to remain vigilant and avoid repetitive use of one active ingredient by using residual herbicides in a program approach to control their tough weeds multiple ways.

“We’re trying to manage the expansion of glyphosate-resistant weeds — marestail, waterhemp and giant ragweed, for example,” says Dave Ruen, field scientist, Dow AgroSciences. “There’s no question in my mind that we’ve slowed the advance of glyphosate resistance due to the resurgent use of soil-applied herbicides, such as Sonic in soybeans.”

Some weeds, such as tall waterhemp, are developing stacked resistance to multiple modes of action, an issue farmers must keep their eyes on.

In Kansas, some tall waterhemp plants are now resistant to HPPD inhibitors, ALS inhibitors and atrazine. Meanwhile in Illinois, tall waterhemp is documented with multiple resistance to PPO inhibitors, ALS inhibitors and atrazine.

“In the last few years, we’ve seen a slow expansion of herbicide-resistant weeds, particularly in the upper Midwest,” Ruen says. “It’s important that we continue to increase the use of residual herbicides and not skimp on rates.”

Bill Johnson, professor of weed science at Purdue University, agrees. Johnson says the majority of successful weed control programs involve a preemergence, residual herbicide. As new postemergence growth-regulator herbicides enter the market, they will provide better control than glyphosate on glyphosate-resistant populations, but it’s not going to improve control from zero percent to 100 percent.

“We have to be realistic in our expectation,” Johnson says. “So if we get into a situation with only two shots of 2,4-D or dicamba postemerge, what’s going to happen three, four, five years down the road? We’re going to have resistance because we’re basically spraying that weed with one active ingredient. Because of the pricing situation with soybeans, I know that’s what the temptation is going to be, but we want to warn against that.”

Four best management practices for clean fields

To stop weeds from robbing yield, carefully scout soybean fields and use a targeted program approach this season.

Control weeds early — Use full rates of a broad spectrum, preemergence herbicide that targets the herbicide-resistant weeds causing the most concern; Apply multiple modes of action that have powerful activity on the Amaranthus species — pigweeds, waterhemps, Palmer and Powell amaranth; with two nonglyphosate modes of action, Sonic is proven to provide 94 percent control of waterhemp and 93 percent control of Palmer amaranth.

Follow with a timely postemergence herbicide — Choose the appropriate herbicide(s) that will effectively control any remaining weeds; Be ready to follow up with a sequential postemergence application as needed. Strive for perfect weed control to reduce the weed seed bank in the soil.